The 3 in 1 does NOT use a memory card. it is an expansion pack; all it does is temporarily store a game to it using a section of memory called the PSRAM, and you can save ONE GAME to the other section of memory called the NOR (unless you own an EZ Flash V, then you can save a few games with an old firmware).

But yeah, the 3 in 1 is more of a way to just play games from a Slot 1 card; it's not the best solution to play in a GBA. I have one and I only use it to let friends play MOTHER 3 now, since I have a EZ Flash IV I never really use the 3 in 1 much.

The EZ-Flash IV can be used as generic 2in1 version of the 3in1 if you have a slot1 flashcart and gba exploader.

Warning: Spoilers inside!

It can in some cases wipe out the EZ-Flash IV loader in some manner not known yet. In which case you would have to be able to flash a new bin loader to the EZ-Flash IV to fix and erase the NOR contents if any.

There is an actual EZ-Flash IV 2in1 loader you can use. Limits you to the PSRAM and RAM mode only (slot1 app).